The present invention relates to a new offset printing method and a printing plate for use in a usual light-load printing field, particularly in an offset printing operation, and more particularly to an offset printing method and a printing plate capable of easily making a printing plate. More particularly, the present invention relates to an offset printing method with which a printing plate can repeatedly be reused. The present invention also relates to the above printing plate and a printing apparatus using the above printing method.
The offset printing method has a process for making a printing plate which is very simple among a multiplicity of printing methods. Therefore, the offset printing method has usually been employed as a main printing means present. The foregoing printing technique is structured to use the immiscibility between oil and water. An oil material, that is, ink is selectively maintained in an image region, while dampening water is selectively maintained in non-image regions. Therefore, direct contact with a surface on which an image will be printed or contact with the same through an intermediate medium, called a xe2x80x9cblanketxe2x80x9d causes ink in the image portion to be transferred. Thus, printing is performed.
The offset printing operation is mainly performed by a method using a PS plate incorporating a support member which is an aluminum substrate and on which a diazo photosensitive layer has been formed by coating. The PS plate is configured such that the surface of the aluminum substrate, which is the support member, is grained, subjected to anode oxidation and other processes to improve the receiving performance and a repulsion characteristic of the non-image portion against ink. Thus, printing resistance is raised and the printing surface is made to be precise. Then, an image, which must be printed, is formed on the printing surface. Therefore, the offset printing has characteristics including simplicity, printing resistance and precise printing surface.
Since the precise characteristic has been attained to the offset printing method, the offset printing method has widely been used in a usual printing field. On the other hand, the offset printing method has been required to have a furthermore simplified structure. Thus, a variety of simple offset printing methods have been suggested.
A representative method of the simple offset printing method is a method using xe2x80x9cCopyrapidxe2x80x9d offset printing plate marketed by Agfa-Gevaert Ltd. Moreover, printing methods of the foregoing type have been disclosed in U.S. Pat. No. 3,511,656 and Japanese Patent Publication No. 7-56351A. The disclosed method is arranged to make a printing plate by a silver-salt diffusion transfer method. The foregoing method is able to form an image, which must be transferred, in only one step. Moreover, the image, which must be transferred, has a lipophilic nature. Therefore, the foregoing image can directly be used as the printing plate. Hence it follows that the foregoing printing method has been put into practical used as a simple printing method. However, the foregoing simple method requires a diffusion transfer development step using alkaline developing solution. Therefore, there is a requirement for a simple printing method which does not require a developing step using developing solution.
A method of making a simple printing plate in which the developing step using the alkaline developing solution and arranged to be performed after exposure of an image is omitted has been developed under the foregoing circumstances. In the technical field of the simple printing plate also called an xe2x80x9cunprocessed platexe2x80x9d because the developing step can be omitted, a variety of means for forming an image on a recording surface has been suggested which are based on a variety of principles as follows: (1) an irradiated portion of the surface is broken due to heat of exposure; (2) an irradiated portion has a lipophilic nature by the exposure; (3) an irradiated portion is harden due to a light mode to have a lipophilic nature; (4) surface nature is changed due to light-decomposition of diazo compounds; and (5) fusible thermal transfer using a heat mode, and so on.
Disclosed techniques adaptable to the foregoing simple offset printing method include a technique disclosed in U.S. Pat. No. 3,506,779, U.S. Pat. No. 3,549,733, U.S. Pat. No. 3.574,657, U.S. Pat. No. 3.739,033, U.S. Pat. No. 3,832,948, U.S. Pat. No. 3,945,318, U.S. Pat. No. 3,962,513, U.S. Pat. No. 3,964,389, U.S. Pat. No. 4,034,183, U.S. Pat. No. 4,081,572, U.S. Pat. No. 4,693,958, U.S. Pat. No. 4,731,317, U.S. Pat. No. 5,238,778, U.S. Pat. No. 5,353,705, U.S. Pat. No. 5,385,092, U.S. Pat. No. 5,395,729 and European Patent No. 1068.
Each of the foregoing techniques has been devised such that no developing solution is required when the printing plate is made. Each of the techniques has one or more problems which include insufficient difference between lipophilic regions and hydrophilic region, causing the image quality of the printed image is unsatisfactory, insufficient resolution which a printed image exhibiting excellent sharpness cannot be obtained, insufficient mechanical strength of the image surface and, therefore, occurrence of easy damage of the image surface, deterioration in the simplicity caused from provision of a protective film to prevent the damage and insufficient durability to endure printing for a long time. Therefore, simple omission of the alkali developing step cannot improve the degree of practical use. The requirement for a method of making a printing plate satisfying a variety of characteristics required for a printing operation and capable of simply making a printing plate has not been met in spite of a plurality of the foregoing improvements.
As one of a method of making the unprocessed printing plate, a method of making a printing plate has been disclosed in Japanese Patent Publication No. 9-169098A which uses a fact that zirconia ceramic is caused to have a hydrophilic nature owing to irradiation with light. The photosensitivity of zirconia, however, is insufficient and an unsatisfactory photoconversion effect from the hydrophobic nature to the hydrophilic nature inhibits easy identification between image portions and non-image portions.
If means capable of easily reuse and reusing the used printing plate can be employed in addition to the simple printing method which does not require the developing solution, an advantage can be obtained in that the cost and the quantity of waster can be reduced. When the printing plate is reused, the reusing operation must easily be performed. It is difficult to structure a simple reusing operation. There has substantially no investigation has been performed. Only a special material for the printing plate, which is zirconia ceramic, has been disclosed in Japanese Patent Publication No. 9-169098A.
In view of the foregoing, the first object of the present invention is to provide an offset printing method which does not require alkali developing solution, which is capable of easily making a printing plate, which exhibits a satisfactory identifying characteristic between image portions and non-image portions on the printing surface, which enables a printed matter having a practically required quality to be obtained, and which permits repeated use of the printing plate.
The second object of the present invention is to provide a printing apparatus adapted to the foregoing printing method, capable of printing an image having a practically required image quality level and permitting repeated use of the printing plate.
The third object of the present invention is to provide a printing method for making a negative-type plate making method and satisfying the above-mentioned objects.
As the first aspect of the invention, the present inventors have found that the surface of metal oxide and metal of a certain type has a characteristic that the degree of the hydrophobic nature and the hydrophilic nature is changed owing to action of heat. Another fact has been found that the foregoing change is caused to arbitrarily occur to the hydrophobic nature or the hydrophilic by changing the heating conditions. The foregoing characteristic is used to form an image, which must be formed, on the surface of the printing plate and to erase the image on the surface of the printing plate after the printing operation has been completed. When the foregoing method is used, a fact has been found that the foregoing problem can be overcome.
The first aspect of the present invention is based on the discovery of a special behavior of the physical properties of the surface of a specific substance (mainly the foregoing metal oxide and metal) occurring with heat. That is, the clear surface of the specific substance originally has a hydrophilic nature. If the foregoing surface is heated at (1) an adequate temperature (hereinafter called a xe2x80x9chydrophobicity developing temperaturexe2x80x9d), the characteristic is changed to the hydrophobic nature. (2) When the surface is heated at a higher temperature (hereinafter called a xe2x80x9chigher hydrophilicity developing temperaturexe2x80x9d), the surface again has the hydrophilic nature. Moreover, (3) the foregoing change in the characteristic of the surface has a hysteresis nature.
The foregoing characteristic is used such that a first step is performed so that the surface of the substance is caused to have the hydrophilic nature by heating the substance at the temperature at which hydrophilicity characteristic is developed. Then, a second step is performed so that the surface of the substance corresponding to the image is heated at the hydrophobicity developing temperature so that a hydrophobic region is formed to correspond to the image. Then, a third step is performed so that printing ink is held in the hydrophobic region and dampening water is held in the hydrophilic region. Thus, offset printing can be performed. After the printing operation has been completed, ink on the used printing plate is removed by cleaning. The foregoing printing plate is again heated to the temperature at which hydrophilicity is developed so that the printed image is removed. As a result, the foregoing printing plate can again be used in the plate making step and the printing step. As a matter of course, the temperature of the surface of the substance must be lowered to a level not higher than hydrophobicity developing temperature between the step for heating the surface at the temperature at which hydrophilicity is developed in the first step and the step for heating the substance at the hydrophobicity developing temperature in the second step. The substance applied to the present invention has a hysteresis nature. Therefore, the surface once caused to have the hydrophilic nature at the higher hydrophilicity developing temperature is cooled without change to the hydrophobic nature if the temperature is made to be the hydrophobicity developing temperature during the cooling process. When the hydrophilic surface cooled to the level not higher than the hydrophobicity developing temperature is again heated to the hydrophobicity developing temperature, the surface is caused to have the hydrophobic nature. Moreover, the hydrophobic image region obtained on the printing plate owing to the recording operation performed in the heat mode can stably be maintained thanks to the hysteresis nature even if the temperature is not higher than room temperatures. In the following description, the foregoing substance having the characteristics (1), (2) and (3) with respect to heat is called a xe2x80x9cthermal responsive substancexe2x80x9d. The thermal responsive substances are widely detected among metal and metal oxide. The foregoing metal and metal oxide are called xe2x80x9cthermal responsive metalxe2x80x9d and xe2x80x9cthermal responsive metal oxidexe2x80x9d, respectively. The thermal responsive substance and its singular behavior will be described later.
As the second aspect of the present invention, the inventors have confirmed existence of a substance which has a photocatalyst function and which displays special change behavior of the hydrophobic nature/hydrophilic nature depending on the temperature and the heating conditions when the substance is heated. The foregoing characteristics are used to form an image which must be printed and which is formed on the surface of the printing plate and to erase the image on the surface of the printing plate after the printing operation has been completed so that the foregoing is overcome.
The second aspect of the present invention is based on the discovery of a fact that the surface of a specific substance, such as titanium oxide, is changed to a hydrophilic nature when the substance is irradiated with light having a specific wavelength. The foregoing substance is called a substance having a photocatalyst function. Light having the specific wavelength is called activation light. Moreover, a fact has been found that the some thermal responsive substances described above are included in the substances having the photocatalyst function.
The photocatalyst function and the thermal responsive characteristic are used such that a first step is performed so that the surface of the substance is caused to have the hydrophilic nature by irradiating the surface of the substance with activation light. Then, a second step is performed so that the surface of the substance corresponding to the image is heated at the hydrophobicity developing temperature so that a hydrophobic region is formed to correspond to the image. Then, a third step is performed so that printing ink is held in the hydrophobic region and dampening water is held in the hydrophilic region. Thus, offset printing can be performed. After the printing operation has been completed, ink on the used printing plate is removed by cleaning. The foregoing printing plate is again irradiated with activation light so that the printed image is removed. As a result, the foregoing printing plate can again be used in the plate making step and the printing step. The surface of the substance, such as the metal oxide, which has the photocatalyst function and as well as the thermal response characteristic, is made to have the hydrophilic nature owing to irradiation with light. The hydrophilic nature of the surface is maintained for a sufficiently long time from a viewpoint of practical use thanks to the hysteresis nature. Also the hydrophobic nature of region of the hydrophilic surface heated to the hydrophobicity developing temperature to correspond to the image is stably maintained at room temperatures thanks to the hysteresis nature. Therefore, distribution of the hydrophobic and hydrophilic portions corresponding to the image can be used in printing and plate making steps.
According to the third aspect of the present invention, there are provided a negative offset printing method and a printing apparatus which effectively use the photocatalyst function and the thermal response characteristic.
The printing method is performed such that a first step is performed so that the surface of the substance is caused to have the hydrophilic nature by uniformly heating the surface of a specific substance which performs the photocatalyst reaction at the temperature at which hydrophobicity is developed to cause the surface of the substance to have a hydrophobic nature. Then, a second step is performed so that the surface of the substance is irradiated with activation light to correspond to an image to form distribution of hydrophilic regions and hydrophobic regions corresponding to an image. Then, a third step is performed so that printing ink is held in the hydrophobic region and dampening water is held in the hydrophilic region. Thus, offset printing can be performed. After the printing operation has been completed, ink on the used printing plate is removed by cleaning. The foregoing printing plate is again heated at the temperature at which hydrophobicity is developed. The distribution of hydrophilic regions and hydrophobic regions corresponding to an image is erased so that a uniform hydrophobic surface is obtained. As a result, the foregoing printing plate can again be used in the plate making step and the printing step. The surface of the substance, such as metal and metal oxide, having the photocatalyst function has the different degrees of the original hydrophilic nature or the hydrophobic nature depending on the type of the substance. Also the foregoing degree varies depending on the elapsed time. In a case of the substance having the thermal response characteristics (1) to (3), when the surface is heated at the temperature at which hydrophobicity is developed to have the hydrophobic nature, the hydrophilic nature of the surface is maintained for a sufficiently long time from a viewpoint of practical use thanks to the hysteresis nature. Moreover, the present invention is able to adequately control the temperature in the hydrophobic nature imparting process in which heating in the first step is performed in such a manner that the temperature is not raised to the high temperature at which hydrophilicity is developed. Therefore, the hydrophobic nature can be enhanced under an optimum temperature condition.
According to the fourth aspect of the present invention, there are provided a negative offset printing method and a printing apparatus which effectively use a fact that the surface of metal oxide and metal of a certain type has a characteristic that the degree of the hydrophobic nature and the hydrophilic nature is changed owing to action of heat, and another fact that the foregoing change is caused to arbitrarily occur to the hydrophobic nature or the hydrophilic by changing the heating conditions.
The printing method is performed such that a first step is performed so that the surface of the substance is caused to have the hydrophilic nature by heating the substance at the temperature at which hydrophilicity characteristic is developed. Then, a second step is performed so that the surface of the substance corresponding to the image is heated at the temperature at which hydrophobicity is developed so that a hydrophobic region is formed to correspond to the image. Then, a third step is performed so that printing ink is held in the hydrophobic region and dampening water is held in the hydrophilic region. Thus, offset printing can be performed. After the printing operation has been completed, ink on the used printing plate is removed by cleaning. The foregoing printing plate is again heated to the temperature at which hydrophilicity is developed so that distribution of image of hydrophilic and hydrophobic natures is erased. Thus, a uniform hydrophobic surface can be obtained. As a result, the foregoing printing plate can again be used in the plate making step and the printing step.